Tubulin is an important target in controlling disease states associated with cell proliferation such as cancer and inflammation (eg, psoriasis). Tubulin is composed of a heterodimer of two related proteins called α and β tubulin. Tubulin polymerises to form structures called microtubules. Compounds that inhibit tubulin's ability to polymerise to form microtubules interrupt cell division which is dependent on the formation of microtubules to form mitotic spindles. Examples of such compounds include the Vinca alkaloids such as vincristine and vinblastine.
Furthermore, compounds that inhibit the depolymerisation of microtubules can also prevent cell division since they often disrupt the proper formation of mitotic spindles which must also disassemble in order for cell division to be completed. Interruption of the mitotic process in this manner often induces cell death by an apoptotic mechanism. Examples of compounds which act in this manner include the taxoids such as paclitaxel.
For these antimitotic agents, selectivity for diseased versus non-diseased tissue is based on relative rates of proliferation, where the diseased tissue more rapidly proliferates. Accordingly, diseased tissue is generally more sensitive to the effect of these agents because it is more likely to be in a state of mitosis which is the stage of a cell's life cycle affected by agents that target tubulin. Unfortunately however, a number of normal, healthy tissues also have quite high rates of proliferation (for example hair follicles and the lining of the gastro-intestinal tract) and accordingly, these tissues can be damaged during chemotherapy with these agents.
Tubulin is also a target for treating disease states that are dependent or result from the abnormal formation of blood vessels (neovascularisation) such as in cancerous tumours and in ocular myopathy. In these cases the cytoskeleton of the vascular endothelial cells are disrupted through depolymerisation of microtubles, which results from inhibiting the polymerisation of tubulin to form microtubules. Microtubule length is dependent on the rate of depolymerisation versus polymerisation. Depolymerising microtubles through inhibition of polymerisation leads to a change in endothelial cell morphology, which then causes a blockage or shutdown in blood flow. In the case of cancerous tumours, blood flow to the diseased tissue is stopped, depriving the tumour of oxygen and nutrients leading to necrotic cell death. Neovascular systems are more sensitive to these agents because they are more dependent on microtubule cytoskeletons than normal, healthy, vascular endothelial cells which are also supported by actin based cytoskeletal structures. For a number of tubulin polymerisation inhibitors (TPIs) that target the colchicine binding site of tubulin, the vascular targeting modality can be achieved at a lower in vivo concentration than the antiproliferative modality. In theory though, agents that target the colchicine binding domain of tubulin are potentially dual mode agents (ie. antimitotic and antivascular).
One of the most potent inhibitors of tubulin polymerisation that binds to the colchicine binding domain of tubulin is the cis-stilbene, combretastatin A4 (CA4) (1). Due to its insolubility CA4 is administered as its prodrug equivalent combretastatin A4 disodium phosphate (CA4P) (2), where the phosphate is rapidly cleaved in vivo. CA4P is currently undergoing phase I and II clinical trials and is the most advanced vascular targeting agent being trialed. In view of some of the draw-backs associated with CA4P, such as, instability (can isomerise to the inactive trans-stilbene), toxicity and rapid clearance, a number of synthetic groups have sought to prepare more stable analogues that could be designed to exhibit an improved therapeutic index and exhibit improved pharmacokinetics. Recently, a number of TPIs have been identified that contain the benzofuran, indole or benzothiophene ring systems (3). Such ring systems are quite stable and should over come the stability issues associated with CA4P. Unfortunately, such compounds only exhibit moderate tubulin binding and anti-mitotic activity. Accordingly, there exists a need to identify other compounds which are more stable than CA4 and exhibit satisfactory pharmacological properties and/or activity.
